- notes or overview on the UMTS Evolved HSPA MIMO and HSDPA-MIMO used to provide significant improvements in data rate by utilising multipath propagation.

MIMO is one of the major features that have been incorporated into Evolved HSPA / HSPA+.

UMTS HSPA MIMO enables the peak data rates achievable to be increased through the use of what may be termed multi-stream transmission.

Evolved HSPA MIMO basics

MIMO used with HSPA+ is able to provide significant increases in data throughput. With data throughput being one of the key advantages of Evolved HSPA, MIMO is a significant element of the overall system.

Note on MIMO:

Two major limitations in communications channels can be multipath interference, and the data throughput limitations as a result of Shannon's Law. MIMO provides a way of utilising the multiple signal paths that exist between a transmitter and receiver to significantly improve the data throughput available on a given channel with its defined bandwidth. By using multiple antennas at the transmitter and receiver along with some complex digital signal processing, MIMO technology enables the system to set up multiple data streams on the same channel, thereby increasing the data capacity of a channel.

Click on the link for a MIMO tutorial

Traditionally radio systems wanted to minimise the effects of multipath transmission as the multiple paths introduced phase delays that caused distortion and interference. However MIMO exploits the multiple paths in ways that enable the system to become more resilient to interference or to enable high data throughput rates by using spatial multiplexing.

MIMO is able to achieve high data rates - above those predicted by Shannon for a single channel because it is able to utilise the multiple paths to transmit multiple data streams in parallel. However to achieve these high data rates, a correspondingly high carrier to interference ratio must exist for the receiver. This means that spatial multiplexing is really applicable to small cells or larger ones where the receiver, in this case the mobile handset is relatively close to the base station.

In cases where a sufficiently high signal to noise ratio for spatial multiplexing cannot be achieved, the multiple antennas can be used to give receive diversity to improve the reception of a single data stream.


One of the main areas where the increase in data is required is within the downlink. For this the MIMO capability is applied to the HSDPA elements of the signal. The scheme used for HSDPA-MIMO is sometimes referred to as dual stream transmit adaptive arrays - there may be up to two streams of data. HSDPA MIMO is a multi-codeword scheme that uses rank adaptation and pre-coding.

The two streams of data within HSDPA-MIMO are subject to the same physical layer processing, spreading, etc. The same channelisation codes can used to save on channelization code resources.

After this has been completed, linear pre-coding is applied to the signal before the resulting signals are applied to the two antennas. The linear pre-coding attempts to make the two signals nearly orthogonal to each other. This reduces the level of interference between the two signals and also reduces the level of receiver processing required.

In order to support dual stream transmission format of HSDPA-MIMO, the HS-DSCH is modified to support up to two transport blocks per TTI - one transport block per stream.

The use of pre-coding provides several benefits:

  • It is important in the case of single stream transmission where pre-coding provides both diversity gain and array gain. Two antennas are used at the transmitter and weights of the two streams adjusted so that the signals add coherently at the receiver.. This results in a higher signal to noise ratio and thereby increases the coverage area for a particular data rate.
  • In the case of two stream transmission pre-coding is used as to enable the receiver to separate the two data streams. If the pre-coding vectors are chosen to be orthogonal, then the two streams will not interfere.

In order to be able to demodulate the signals, the UE needs to be able to estimate the channel characteristics between all the antennas on the base station and those on the UE. This is achieved by transmitting common pilot channels on each physical transmit antenna. By decoding the pilot channel the UE is able to estimate the channel characteristics.

HSDPA-MIMO rate control

Rate control for HSDPA MIMO is very similar to the rate control used for the standard non-MIMO case. However the system needs know how many streams are being transmitted and also the pre-coding matrix being used. The rate control mechanism needs to determine the number of streams to transmit, the modulation scheme, and the pre-coding matrix. This information is transmitted to the UE on the HS-SCCH.

The multi-stream transmission provided by HSDPA-MIMO is only really beneficial when the signal channel is good, i.e. a high signal to noise ratio. It is therefore only used for the highest data rates. For lower data rates single stream transmission is generally used with the two transmission antennas providing diversity transmission to improve the fading, signal quality, etc.

By Ian Poole

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